1. Technical Field
The present invention relates to a fabrication technique of a piston for an internal-combustion engine. In particular, the present invention relates to a method for producing a piston for an internal-combustion engine having an annular cavity for flowing cooling liquid (cooling oil) into a crown to be exposed to high temperatures.
Priority is claimed on Japanese Patent Application No. 2007-241175, filed Sep. 18, 2007, the content of which is incorporated herein by reference.
2. Background of the Related Art
In general, a piston for use in an internal combustion such as a diesel engine and a petrol engine is made of an aluminum alloy or iron. In the case of a large size diesel engine, iron forging is preferred to withstand its heavy loads. However, in a piston made of iron, a temperature of a top surface of its crown reaches as high as about 500° C. during operation. Even in the case of a piston made of aluminum, the temperature still reaches as high as about 350° C. Since the temperature also becomes high at the periphery of a groove (ring groove) into which a piston ring is mounted, an annular cavity for flowing cooling liquid is generally provided inside the ring groove in order to avoid adverse effects from overheating of the piston ring.
The annular cavity can be formed in casting using a readily-disintegrative core that is made of sand, however, the operation of forming the core or removing the core from the inside of the casting after the core is disintegrated is not easy, resulting in time loss and cost increase.
Hence, after a crown and a skirt of a piston body are formed individually in sections by forging, the parting surface between the crown and the skirt is joined by friction welding, whereby an annular cavity is formed (for example, Japanese Unexamined Patent Application Publication No. 2001-107803).
Furthermore, after an annular groove is formed at the top surface or the upper outer circumference surface of a crown and a rib is formed in connection with the open edge of the annular groove, the rib is bent toward the opening of the annular groove by pressing using a pressure roller, whereby an annular cavity is formed (for example, Japanese Unexamined Patent Application Publication No. 2006-46320).
According to Japanese Unexamined Patent Application Publication No. 2001-107803, this produces the effect of producing a piston body more efficiently compared to an integrated molding using a core. However, when the crown is press-welded to the skirt to thereby form an annular cavity therebetween, a curled burr is generated inside the annular cavity due to friction welding, which narrows the volume of the annular cavity. As a result, flowability of cooling liquid may be lowered, whereby the crown and the skirt of the piston may be burnt or knocking may occur due to overheating of the crown.
In Japanese Unexamined Patent Application Publication No. 2006-46320, since score lines (joint areas between the opening and the rib of the annular groove) may occur during the formation of the annular cavity in the top surface or the upper outer circumference surface of the crown, an inappropriate machining may cause the ejection of cooling liquid in the annular cavity into the combustion chamber through the score lines.
Furthermore, since the top surface and the upper outer circumference surface of the crown are exposed to high temperature and high pressure, high concentrations of stresses may occur at the score lines due to high temperature and high pressure, resulting in breakage of the piston.
The present invention has been made in view of the aforementioned circumstances, and an object of the present invention is to provide a method for efficiently producing a long-life, high-quality piston for an internal-combustion engine having an annular cavity into which cooling liquid can flow effectively.